1. Field of the Invention
The present invention relates to a heat storage tank and, particularly, to a structure of the heat storage tank.
2. Description of the Related Art
A structure of a heat storage tank has been proposed in Japanese Unexamined Patent Publication Nos. 2002-188442 and 2000-73764. The proposed heat storage tank includes a tank body for storing coolant for an internal combustion engine while maintaining the warmth thereof and a housing provided with a fluid passage communicated with the interior of the tank body, through which the coolant passes. The tank body has an inner tank and an outer tank, and a substantial vacuum is formed between the two. The tank body has an port at the lower end thereof for allowing the coolant in the housing to flow in or out from the tank body. This tank body port is inserted and fitted into the housing so that the housing radially encircles the tank body port from outside.
In the heat storage tank, the inner tank and the outer tank are welded together at the lower end of the tank body port. The welded portion is encircled by the housing from radially outside of the tank body port, and if water enters a gap between the welded portion and the housing, the water is difficult to remove. Also, even if the housing is of a structure that the housing is inserted into the tank body port from the inner circumference side so that the welded portion is not encircled by the housing, water entering the lower end of the welded portion is difficult to remove.
If water, particularly water containing salt is retained, the generation of rust becomes problematic. Even if the inner tank and the outer tank are constructed of stainless steel, the welding material is different from stainless steel. Therefore, if outer water is retained between the welded portion and the housing, the welded portion rusts, and there is a risk in that air penetrates the interface of the welded portion generating rust due to long term use, which may lower the degree of vacuum in a space between the inner and outer tanks and degrade the heat retaining property of the heat storage tank.